A Brazilian study investigating whether the anti-malaria drug chloroquine was effective in treating patients with Covid-19 has been halted on safety concerns after a high dose of the drug proved lethal for some patients.

Chloroquine, and a related drug, hydroxychloroquine, in combination with the antibiotic azithromycin, has been touted as a potential treatment for coronavirus by Donald Trump despite a lack of evidence, reports The Guardian.

The termination of the study comes the same day as a letter in the journal Natureraised alarm about serious heart risks associated with Covid-19 patients taking the drugs and the US Food and Drug Administration warned of serious heart complications.

Donald Trump has previously urged Americans to take hydroxychloroquine, although he has apparently backed away from the drug in recent days, and on Thursday even suggested that injections of disinfectant could cure Covid-19 – an idea that was swiftly refuted by experts, who warned the public “please don’t inject bleach”.

The Brazilian study, published in the Journal of the American Medical Association Network Open (Jama), was halted by a safety monitoring board before even one-quarter of the planned 440 patients were enrolled. Eighty-one patients with Covid-19 or suspected Covid-19 were given a daily regimen of chloroquine. The randomized clinical trial separated the patients into a high-dose and a low-dose group.

Researchers planned to assess their outcomes after 28 days.

But after 13 days, six of 40 patients in the low-dose group had died, compared with 16 of 41 patients in the high-dose group. Furthermore, five patients in the high-dose group had underlying heart disease, three of whom died.

“Despite these discouraging findings, several other observations prevent concluding categorically that high-dose chloroquine was toxic,” authors of a comment article said.

Hydroxychloroquine and chloroquine in combination with azithromycin were first put forward as potential treatments for Covid-19 in a French study described as “meaningless”. The journal later said the study did not meet standards for publication.

But by the time the journal disavowed the study’s flawed design, media outlets had already promoted the unproven and risky drug combination. The treatment was tweeted by the tech billionaire Elon Musk and picked up by Fox News. It was then touted by Trump as a potential “game-changer” despite serious risks and lack of safety and efficacy data. At least one man has died and a woman was hospitalized after taking chloroquine.